CISCO DPC3008 Modem Does Not Connect Router to Internet

Comcast provided a CISCO DPC3008 Modem as part of their internet contract. When used 'standalone', it connects a single device to the internet via ethernet without issue. I am unable to connect a router.

I have tried three routers that, as configured, connect from 3 other comcast connections. I just plug them in and they work (I keep a few as loaners if someone has a device fail). The STATUS in each of the routers says they are unable to connect to the internet.

The information I've found in forums suggests there are connection issues, but nothing has been specific to working with wired or wireless routers.

It's fair to say I've setup a few hundred connections that involve modem(s) and router(s) (or combo devices) over the years. This is the first time I've seen this problem. I have: power-cycled, allowed 5 minutes in power cycle, checked for updated firmware, tried using a static IP to the current working one, (though its been ages since Comcast handed out one that specifically needed one) checked for bridge mode.

I am curious if anyone knows of something specific with this device that might help. My inclination, as this is a leased device, is to get a replacement modem.

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That modem is also a router. The user guide for that device suggests connecting a switch or hub to share the connection and not another router. The guide also suggests connecting via a cross over cable.

The troubleshooting section mentions re-obtaining a DHCP lease if a device connected to the cisco device is not able to connect to the internet.

Have read your various insightful replies, noting that you said when the unit is hooked directly to a computer, the computer can go out to the internet, I would start with the cables. You know both the Cisco unit and the Routers work independently. You also have Known MAC addresses...hook everything up and see what you can ping from the computer.

You're right about that, William, and I do hate making these connections. Unfortunately, it does not have wireless capability and that is what we needed. By modifying the DHCP address scheme in the wireless router, I've usually had no trouble making these work. I also tried the crossover cable.

One thing I tried in addition to re-obtaining a DHCP lease was setting a static IP address for the modem-to-router section of the trip. That did not work either.

Looking at your response, James, I tried the one router at home that I had not plugged in there just for grins. It had the same cables I'd finally used on site (I tried about half dozen cables there in addition to the crossover I mentioned in the response to William .. the ones left on the device were all new so I hadn't tried them elsewhere). Worked like a charm.

Since you mentioned MAC addresses, I also entered the MAC addresses manually for the modem into the wireless router but that didn't work either.

With everything hooked up, the ping stopped between the two routers from both sides (connected via a wired port on the modem and attempting to go from the wireless router but from a wired port on the wireless unit).

If wireless is what you are looking for, connect the modem to the LAN side port of your wireless access point. That way you are using the built in switch on your wifi access point so you will have wired and wireless access for the client machines. This also eliminates any DHCP confusion or NAT behind NAT issues that using the WAN side port would create.

This method meets the Cisco device requirement of connecting a switch instead of a router, provides wireless access, allows all devices to be on the same subnet using the same DHCP server, and according the the manufacturers instructions, should work.

Note: Be sure to turn off the DHCP server on your wireless access point so your clients can use the DHCP from the Cisco device.

Two questions:
- "I have tried three routers that, as configured, connect from 3 other comcast connections." What are these routers (make/model)?
- Are you able to configure the DCP3008 to bridge mode?

At the point of having tried all of these troubleshooting steps, I would normally bring in the BIG hammer.....but Comcast won't switch out the unit if you bust it up.

If I had the time to waste, I would get a Comcast Tech online and troubleshoot with them....the troubling point in this is getting out to the internet when the computer is hooked straight to the Cable Modem. I don’t know if this will help, but whatever you hook up to the Comcast device will have to throw IP addresses, as that Modem will not.

I guess it wouldn't be have to be a 110 VAC source to get the job done....
I hope I wasn't misunderstood When I said the Cisco won't throw IP Adresses. It will hand out ONE, and it will go to the first device in line. So, the router that you hook to the Cisco Device will have to hand out IP Addresses to the rest of the systems hooked to it.

William - That's what I was trying, and I tried setting the wireless device both as router and as AP. Neither worked. You are right ... it should have worked.

Marty - Tried that.

Andrew - I've done that with 2 of the 3 before and tried that ... and it didn't work with either.

Steve - DLink DIR615, Belkin N150, Netgear at other location right now so I can't pull the model for you. I think it is a WGR614. I looked in the guide for 'bridge mode' (and mentioned that above so people would know I was familiar with it) and there was no mention of it. I've used it a lot with DSL setups but rarely with Comcast.

James - Too bad they don't warrant against big hammer fixes, isn't it? I wasted almost an hour getting transferred between departments at Comcast ... no one seems to want to own this.

William (re RJ45 trick) - LOL!!!

James (re throwing IP addresses) - And I was getting ready to call Waste Management for help!

Dan - Tried it on the DLink and Belkin. Didn't work on either of those so I didn't try the Netgear.

....

It sounds like making Comcast swap this out is the most reasonable course for the sanity of those concerned. Some day, when I've nothing else to do ...

Debra,
Sorry I do not have one to play with a the moment, but check this.
Make sure your router is on the same IP network as the the modem. Modem
default is 192.168.100.1, so give the router an IP of 192.168.100.2 and
make sure it is exclude from the DHCP range.
Dan

Dan - as I had this working by ethernet connection, we weren't panicked to swap it. We turned it into Comcast, as it was leased from them, and got a unit that works. I was surprised that configuring it as an AP didn't work, so maybe there was an issue with this unit ... just can't prove it.

Dan - as I had this working by ethernet connection, we weren't panicked to swap
it. We turned it into Comcast, as it was leased from them, and got a
unit that works. I was surprised that configuring it as an AP didn't
work, so maybe there was an issue with this unit ... just can't prove it

As I have found out, Comcast, even if a unit is not fully functional but the basic need is covered, will send it out in hopes of not seeing it back. I have recieved some pretty worn units that worked for the first use I had for them, but failed anything past that.